As the DC power supply, there is an AC-DC converter which includes a diode bridge circuit which rectifies an AC power supply and an isolated DC-DC converter which steps down DC voltage rectified by the above circuit and converts the DC voltage to a DC voltage with a predetermined potential. As an AC-DC converter, for example, there is known a switching power supply device in which a switching element connected in a series with a primary side winding of the voltage converting transformer is driven on and off with a PWM (pulse width modulation) control method or PFM (pulse frequency modulation) control method to control the electric current flowing in the primary side winding so that the voltage induced to the secondary side winding is indirectly controlled.
There is also a switching control type AC-DC converter as follows. In order to control operation of a primary side, a resistance for detecting an electric current is provided in a series with a primary side switching element. Also, a terminal (electric current detecting terminal) is provided in a power supply control circuit (IC), and voltage which is converted by electric current-voltage conversion with the resistance is input in the terminal. The peak electric current of the primary winding is controlled based on the detected electric current value and the feedback voltage from the secondary side. With this, the output voltage or the output electric current of the secondary side is maintained at a certain level (see Patent Document 1).
Further, in an AC-DC converter which controls output of the secondary side in the primary side according to voltage converted by electric current-voltage conversion with the resistance for detecting the electric current and feedback voltage from the secondary side, in order to reduce switching loss and enhance power efficiency when the load is light, control is performed in a PWM method with the switching frequency fixed in the region equal to or less than the feedback voltage VFB1 and equal to or more than the VFB2, and control is performed varying the switching frequency between the VFB1 and VFB2.